"speed, because it is less affected by its star's gravity. No planet's orbit is perfectly circular, and hence the distance of each varies over the course of its year. The closest approach to its star is called its periastron (perihelion in the Solar System), whereas its farthest separation from the star is called its apastron (aphelion). As a planet approaches periastron, its speed increases as it trades gravitational potential energy for kinetic energy, just as a falling object on Earth accelerates as it falls; as the planet reaches apastron, its speed decreases, just as an object thrown upwards on Earth"
"(red dot) points of the inner and outer planets. Apsis An apsis (; plural apsides , Greek: ἁψῖδες) is an extreme point in the orbit of an object. The word comes via Latin from Greek and is cognate with ""apse"". For elliptic orbits about a larger body, there are two apsides, named with the prefixes ""peri-"" () and ""ap-""/""apo-"" () added to a reference to the body being orbited. A straight line connecting the periapsis and apoapsis is the ""line of apsides"". This is the major axis of the ellipse, its greatest diameter. The center of mass, or barycenter, of"
"in 2000. The term ""perigee-syzygy"" or ""perigee full/new moon"" is preferred in the scientific community. Perigee is the point at which the Moon is closest in its orbit to the Earth, and syzygy is when the Earth, the Moon and the Sun are aligned, which happens at every full or new moon. Astrophysicist Fred Espenak uses Nolle's definition but preferring the label of ""full Moon at perigee"". Wood used the definition of a full or new moon occurring within 24 hours of perigee and also used the label ""perigee-syzygy"". Wood also coined the less used term ""proxigee"" where perigee and"
"Near-Earth object A near-Earth object (NEO) is any small Solar System body whose orbit brings it to proximity with Earth. By convention, a Solar System body is a NEO if its closest approach to the Sun (perihelion) is less than 1.3 astronomical units (AU). If a NEO's orbit crosses the Earth's and the object is larger than across, it is considered a potentially hazardous object (PHO). Most known PHOs and NEOs are asteroids, but a small fraction are comets. There are over 19,000 known near-Earth asteroids (NEAs), over a hundred short-period near-Earth comets (NECs), and a number of solar-orbiting spacecraft"
"Minimum orbit intersection distance Minimum orbit intersection distance (MOID) is a measure used in astronomy to assess potential close approaches and collision risks between astronomical objects. It is defined as the distance between the closest points of the osculating orbits of two bodies. Of greatest interest is the risk of a collision with Earth. Earth MOID is often listed on comet and asteroid databases such as the JPL Small-Body Database. MOID values are also defined with respect to other bodies as well: Jupiter MOID, Venus MOID and so on. An object is classified as a potentially hazardous object (PHO) –"
"orbit. The orbital ellipse goes through each of the six Earth images, which are sequentially the perihelion (periapsis — nearest point to the Sun) on anywhere from January 2 to January 5, the point of March equinox on March 19, 20, or 21, the point of June solstice on June 20, 21, or 22, the aphelion (apoapsis — farthest point from the Sun) on anywhere from July 3 to July 5, the September equinox on September 22, 23, or 24, and the December solstice on December 21, 22, or 23. The diagram shows an exaggerated shape of Earth's orbit; the"
"Elongation (astronomy) In astronomy, a planet's elongation is the angular separation between the Sun and the planet, with Earth as the reference point. The greatest elongation of a given inferior planet occurs when this planet’s position, in its orbital path around the Sun, is at tangent to the observer on Earth. Since an inferior planet is well within the area of Earth's orbit around the Sun, observation of its elongation should not pose that much a challenge (compared to deep-sky objects, for example). When a planet is at its greatest elongation, it appears farthest from the Sun as viewed from"
"lowest and highest parts of an orbit around Earth, while ""perihelion"" and ""aphelion"" are the closest and farthest points of an orbit around the Sun.) In the case of planets orbiting a star, the mass of the star and all its satellites are calculated to be at a single point called the barycenter. The paths of all the star's satellites are elliptical orbits about that barycenter. Each satellite in that system will have its own elliptical orbit with the barycenter at one focal point of that ellipse. At any point along its orbit, any satellite will have a certain value"
"orbit"" (abbreviated ""AEO"") is a circular areo­synchronous orbit in the Martian equatorial plane about above the surface, any point on which revolves about Mars in the same direction and with the same period as the Martian surface. Areo­stationary orbit is a concept similar to Earth's geo­stationary orbit. The prefix ""areo-"" derives from Ares, the ancient Greek god of war and counterpart to the Roman god Mars, with whom the planet was identified. The modern Greek word for Mars is (). Stationary orbit In celestial mechanics, the term stationary orbit refers to an orbit around a planet or moon where the"
"Orbit In physics, an orbit is the gravitationally curved trajectory of an object, such as the trajectory of a planet around a star or a natural satellite around a planet. Normally, orbit refers to a regularly repeating trajectory, although it may also refer to a non-repeating trajectory. To a close approximation, planets and satellites follow elliptic orbits, with the central mass being orbited at a focal point of the ellipse, as described by Kepler's laws of planetary motion. For most situations, orbital motion is adequately approximated by Newtonian mechanics, which explains gravity as a force obeying an inverse square law."
"between a planet's orbital plane and another plane of reference. For exoplanets, the inclination is usually stated with respect to an observer on Earth: the angle used is that between the normal to the planet's orbital plane and the line of sight from Earth to the star. Therefore, most planets observed by the transit method are close to 90 degrees. Because the word 'inclination' is used in exoplanet studies for this line-of-sight inclination then the angle between the planet's orbit and the star's rotation must use a different word and is termed the spin–orbit angle or spin–orbit alignment. In most"
"Areostationary orbit An areostationary orbit or areosynchronous equatorial orbit (abbreviated AEO) is a circular areo­synchronous orbit in the Martian equatorial plane about above the surface, any point on which revolves about Mars in the same direction and with the same period as the Martian surface. Areo­stationary orbit is a concept similar to Earth's geo­stationary orbit. The prefix ""areo-"" derives from Ares, the ancient Greek god of war and counterpart to the Roman god Mars, with whom the planet was identified. The modern Greek word for Mars is ""Άρης"" (Áris). To date, no artificial satellites have been placed in this orbit,"
"the orientation of the Earth's rotational axis in space. The Earth's orbit plane is called the ecliptic, and it does not coincide with the Earth's equatorial plane. The angle between the Earth's equatorial plane and the ecliptic, ""ε"", is called the obliquity of the ecliptic and ""ε"" ≈ 23.4°. An equinox occurs when the earth is at a position in its orbit such that a vector from the earth toward the sun points to where the ecliptic intersects the celestial equator. The equinox which occurs near the first day of spring (with respect to the North hemisphere) is called the"
"course of its year. A body's closest approach to the Sun is called its ""perihelion"", whereas its most distant point from the Sun is called its ""aphelion"". The orbits of the planets are nearly circular, but many comets, asteroids, and Kuiper belt objects follow highly elliptical orbits. The positions of the bodies in the Solar System can be predicted using numerical models. Although the Sun dominates the system by mass, it accounts for only about 2% of the angular momentum. The planets, dominated by Jupiter, account for most of the rest of the angular momentum due to the combination of"
"Apsis An apsis (; plural apsides , Greek: ἁψῖδες) is an extreme point in the orbit of an object. The word comes via Latin from Greek and is cognate with ""apse"". For elliptic orbits about a larger body, there are two apsides, named with the prefixes ""peri-"" () and ""ap-""/""apo-"" () added to a reference to the body being orbited. A straight line connecting the periapsis and apoapsis is the ""line of apsides"". This is the major axis of the ellipse, its greatest diameter. The center of mass, or barycenter, of a two-body system lies on this line at one"
"Earth is approximately which is close to the correct value (0.016710219) (see Earth's orbit). The calculation is correct when perihelion, the date the Earth is closest to the Sun, falls on a solstice. The current perihelion, near January 3, is fairly close to the solstice of December 21 or 22. It took nearly two centuries for the current formulation of Kepler's work to take on its settled form. Voltaire's ""Eléments de la philosophie de Newton"" (Elements of Newton's Philosophy) of 1738 was the first publication to use the terminology of ""laws"". The ""Biographical Encyclopedia of Astronomers"" in its article on"
"Osculating orbit In astronomy, and in particular in astrodynamics, the osculating orbit of an object in space at a given moment in time is the gravitational Kepler orbit (i.e. an elliptic or other conic one) that it would have around its central body if perturbations were absent. That is, it is the orbit that coincides with the current orbital state vectors (position and velocity). The word ""osculate"" is Latin for “kiss”. In mathematics, two curves osculate when they just touch, without (necessarily) crossing, at a point, where both have the same position and slope, i.e. the two curves “kiss”. An"
"Argument of periapsis The argument of periapsis (also called argument of perifocus or argument of pericenter), symbolized as ""ω"", is one of the orbital elements of an orbiting body. Parametrically, ""ω"" is the angle from the body's ascending node to its periapsis, measured in the direction of motion. For specific types of orbits, words such as perihelion (for heliocentric orbits), perigee (for geocentric orbits), periastron (for orbits around stars), and so on may replace the word periapsis. (See apsis for more information.) An argument of periapsis of 0° means that the orbiting body will be at its closest approach to"
"terms ""inferior conjunction"" and ""superior conjunction"" are used in particular for the planets Mercury and Venus, which are inferior planets as seen from the Earth. However, this definition can be applied to any pair of planets, as seen from the one farther from the Sun. A planet (or asteroid or comet) is simply said to be in conjunction, when it is in conjunction with the Sun, as seen from the Earth. The Moon is in conjunction with the Sun at New Moon. In a quasiconjunction, a planet in retrograde motion — always either Mercury or Venus, from the point of"
"in the WGSN's second bulletin issued in October 2016. The next additions were done on 1 February, 30 June, 5 September and 19 November 2017, and on 6 June 2018. All are included on the current List of IAU-approved Star Names, last updated on 1 June 2018. The star nearest to Earth is typically referred to simply as ""the Sun"" or its equivalent in the language being used (for instance, if two astronomers were speaking French, they would call it ""le Soleil""). However, it is sometimes called by its Latin name, Sol, in science fiction. With the advent of the"
"Heliocentric orbit A heliocentric orbit (also called circumsolar orbit) is an orbit around the barycenter of the Solar System, which is usually located within or very near the surface of the Sun. All planets, comets, and asteroids in the Solar System, and the Sun itself are in such orbits, as are many artificial probes and pieces of debris. The moons of planets in the Solar System, by contrast, are not in heliocentric orbits, as they orbit their respective planet (although the Moon has a convex orbit around the Sun). The barycenter of the Solar System, while always very near the"
"a moon of another planet from its central planet, for instance the angular distance of Io from Jupiter. Here we can also talk about ""greatest eastern elongation"" and ""greatest western elongation"". In the case of the moons of Uranus, one often talks about ""greatest northern elongation"" and ""greatest southern elongation"" instead, due to the very high inclination of Uranus' axis of rotation.. Elongation (astronomy) In astronomy, a planet's elongation is the angular separation between the Sun and the planet, with Earth as the reference point. The greatest elongation of a given inferior planet occurs when this planet’s position, in its"
"Orbital period The orbital period is the time a given astronomical object takes to complete one orbit around another object, and applies in astronomy usually to planets or asteroids orbiting the Sun, moons orbiting planets, exoplanets orbiting other stars, or binary stars. For objects in the Solar System, this is often referred to as the sidereal period, determined by a 360° revolution of one celestial body around another, e.g. the Earth orbiting the Sun. The name ""sidereal"" is added as it implies that the object returns to the same position relative to the fixed stars projected in the sky. When"
"near Earth, Earth radii or kilometers are used. A corresponding right-handed rectangular coordinate system is also used occasionally; the ""x""-axis is directed toward the vernal equinox, the ""y""-axis 90° to the east, and the ""z""-axis toward the north ecliptic pole; the astronomical unit is the unit of measure. Symbols for ecliptic coordinates are somewhat standardized; see the table. Ecliptic coordinates are convenient for specifying positions of Solar System objects, as most of the planets' orbits have small inclinations to the ecliptic, and therefore always appear relatively close to it on the sky. Because Earth's orbit, and hence the ecliptic, moves"
"are rarely, if ever used. Instead, the generic suffix of -apsis is used. The words perihelion and aphelion were coined by Johannes Kepler to describe the orbital motion of the planets. The words are formed from the prefixes ""peri-"" (Greek: ""περί"", near) and ""apo-"" (Greek: ""ἀπό"", away from) affixed to the Greek word for the sun, ἥλιος. Currently, the Earth reaches perihelion in early January, approximately 14 days after the December Solstice. At perihelion, the Earth's center is about astronomical units (AU) or from the Sun's center. In contrast, the Earth reaches aphelion currently in early July, approximately 14 days"
"may be orbiting Sun-like stars. The nearest such planet may be 12 light-years away. Scientific findings since the 1990s have greatly influenced the scope of the fields of astrobiology, models of planetary habitability and the search for extraterrestrial intelligence (SETI). NASA and the SETI Institute have proposed categorising the increasing number of planets found using a measure called the Earth Similarity Index (ESI) based on mass, radius and temperature. According to this measure, as of 23 July 2015, the confirmed planet currently thought to be most similar to Earth on mass, radius and temperature is Kepler-438b. Scientists estimate that there"
"Orbiter Mission. Heliocentric orbit A heliocentric orbit (also called circumsolar orbit) is an orbit around the barycenter of the Solar System, which is usually located within or very near the surface of the Sun. All planets, comets, and asteroids in the Solar System, and the Sun itself are in such orbits, as are many artificial probes and pieces of debris. The moons of planets in the Solar System, by contrast, are not in heliocentric orbits, as they orbit their respective planet (although the Moon has a convex orbit around the Sun). The barycenter of the Solar System, while always very"
"line of apsides of Earth's elliptical orbit. The orbital ellipse (with eccentricity exaggerated for effect) goes through each of the six Earth images, which are sequentially the perihelion (periapsis—nearest point to the sun) on anywhere from 2 January to 5 January, the point of March equinox on 19, 20 or 21 March, the point of June solstice on 20 or 21 June, the aphelion (apoapsis—farthest point from the sun) on anywhere from 4 July to 7 July, the September equinox on 22 or 23 September, and the December solstice on 21 or 22 December. These ""astronomical"" seasons are not of"
"Earth ellipsoid An Earth ellipsoid is a mathematical figure approximating the Earth's form, used as a reference frame for computations in geodesy, astronomy, and the geosciences. Various different ellipsoids have been used as approximations. It is an ellipsoid of revolution whose minor axis (shorter diameter), which connects the geographical North Pole and South Pole, is approximately aligned with the Earth's axis of rotation. The ellipsoid is defined by the equatorial axis ""a"" and the polar axis ""b""; their difference is about 21 km, or 0.335%. Additional parameters are the mass function ""J2"", the correspondent gravity formula, and the rotation period"
"Planet A planet is an astronomical body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals. The term ""planet"" is ancient, with ties to history, astrology, science, mythology, and religion. Five planets in the Solar System are visible to the naked eye. These were regarded by many early cultures as divine, or as emissaries of deities. As scientific knowledge advanced, human perception of the planets changed, incorporating a number of disparate objects. In 2006, the International"
"period in orbit and at the surface. Asteroid belt The asteroid belt is the circumstellar disc in the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets. The asteroid belt is also termed the main asteroid belt or main belt to distinguish it from other asteroid populations in the Solar System such as near-Earth asteroids and trojan asteroids. About half the mass of the belt is contained in the four largest asteroids: Ceres, Vesta, Pallas, and Hygiea. The total mass of the asteroid"
"following are some of the more common ones. A synchronous orbit around Earth that is circular and lies in the equatorial plane is called a geostationary orbit. The more general case, when the orbit is inclined to Earth's equator or is non-circular is called a geosynchronous orbit. The corresponding terms for synchronous orbits around Mars are areostationary and areosynchronous orbits. An astronomical example is Pluto's largest moon Charon. Much more commonly, synchronous orbits are employed by artificial satellites used for communication, such as geostationary satellites. For natural satellites, which can attain a synchronous orbit only by tidally locking their parent"
"parallel to the orbital axis, then the direction the thumb points is defined to be the orbital north. The poles of Earth's orbit are referred to as the ecliptic poles. The ecliptic is the plane on which Earth orbits the Sun. The ecliptic poles are the two points where the ecliptic axis, the imaginary line perpendicular to the ecliptic, intersects the celestial sphere. The two ecliptic poles are mapped below. Due to axial precession, either celestial pole completes a circuit around the nearer ecliptic pole every 25,800 years. , the positions of the ecliptic poles expressed in equatorial coordinates, as"